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Bumblebee Deaths
by Duncan Bridges
22 May 2010
In recent weeks we have had several reports of individual or groups of bumblebees being found dead or dying on the ground. This kind of incident is often reported at this time of year as bumblebee colonies get established.
 There are several possible causes of a group death of bumblebees like this:
Natural death; Worker bumblebees have relatively short lives – 4 weeks being the average time span. When a colony is first established the queen lays a batch of eggs which she broods until they become larvae and almost all of these larvae will become adult bees at about the same time. It is possible therefore that the first batch of adult worker bees in a colony will mostly die off at about the same time, approximately 4 to 5 weeks after the colony is first established.
Other causes of death:
• Cold Weather. One cause is that the weather, turning colder after a reasonable warm period in spring, has caused them to die off if they have been caught out or were unable to return to the nest. Bumblebees can normally survive overnight outside the nest so for this to happen the temperature needs to dip quite suddenly and severely, usually in the early evening.
• Many bumblebee species live as small colonies in burrows in the ground, or in soft material in walls and banks. Having built their nest during a prolonged dry period if there is heavy rainfall the nest might become flooded, the bees become waterlogged and subsequently they may be drowned or killed by cold.
• Another possibility is that they have been predated on by birds – several species of birds, great tits in particular, are recorded as attacking bumblebees - look for damage to the rear end of the bee as the birds often attack the hind end first to remove the bee’s sting.
• A similar scenario is that something has attacked the bees nest, and the bees have responded by attacking the predator, using their stings on the predator but dying in the process. A careful look at the bodies to see if the stings have been used / torn out can be evidence of this and there may be evidence of disturbance to the nest itself
• Another possible cause is that they have been killed by a parasite - there are a number of parasites, grubs and mites that feed internally on bumblebees. Look for damage or cavities in the bee’s body or evidence of the parasite still in situ. Note- there are many species of parasite that will feed on a bee post-mortem so this can often be hard to be clear about unless the bee is observed at the time of death.
• And lastly, sadly, there is always the possibility of poisoning by some means - most frequent vector is a pesticide sprayed on some plant/s that the bees have recently been feeding on. This usually gives no physical evidence, but often causes group deaths of a colony.
So if you find a group of bumblebee’s all dead in the same place it most likely to be the result of natural mortality, attack by predator, poisoning or waterlogging / cold. A careful look at the carcasses can give clues to predator attack, but natural mortality, poisoning or death from cold usually leaves the bodies unmarked and can therefore be hard to determine.
Death caused by parasites more often results in individual rather than group deaths.
Report Group Deaths to the Trust
If you find any groups of bumblebee’s dead in one place, please contact us (email:
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or by phone: 01624 801985) with details such as date, time, location, species (if known) and any evidence of possible cause. Photographs of one or more of the bees will also help.
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